The invention relates to a vehicle brake system, and more particularly, to a brake system with an electro-hydraulic secondary brake system.
Certain jurisdictions, such as Europe, have regulations which require off-road vehicles, such as tractors, to have a secondary brake system to help stop the tractor when the tractor is being operated at higher speeds. All tractors sold in Europe have service brakes which operate on the rear axle and which are controlled by independent foot pedals. A hand-operated lever is typically used to actuate the secondary brake system (the front brakes), But, such systems do not actuate the secondary brakes when both service brake pedals are simultaneously depressed in order to assist in stopping the tractor, and such systems do not vary the application of the secondary brakes as a function of brake pressure.
Accordingly, an object of this invention is to provide a brake system which includes an electrohydraulic secondary brake control valve and an electronic control unit which cooperate to actuate secondary brakes when both service brake pedals are simultaneously depressed in order to assist in stopping the tractor.
Another object of this invention is to provide such a brake system which controls the application of the secondary brakes as a function of brake pressure.
These and other objects are achieved by the present invention, wherein a vehicle brake system includes a primary hydraulic pump, a secondary variable delivery pump, left and right service brakes and left and right secondary brakes. A service brake valve is connected between the primary pump and the service brakes, and operates the service brakes in response to brake pedal operation. Left and right service brake switches generate signals representing operational status of the service brakes. An electrohydraulic secondary brake valve operates the secondary brakes in response to an electronic control signal. Brake pressure sensors generate brake pressure signals representing the service brake pressure and the secondary brake pressure. An electronic control unit is coupled to the service brake switches, to the pressure sensors and to the secondary brake valve. The control unit generates the control signal as a function of the status of the service brakes and of the brake pressure signals. When both service brakes are applied, and the service brake pressure is at a certain level, the control unit causes the secondary brake valve to operate the secondary brakes.